As a structure for mounting a semiconductor device that is made of a packaged semiconductor element on a circuit board, a structure in which a projected electrode such as a solder bump formed on a semiconductor device is bonded to a base plate is known. In a semiconductor device having such a structure, an attempt to make a semiconductor device as thin as possible, that is, 150 μm or less is in progress. This intends, by reducing stress during the heat cycle, to realize high bonding reliability after the mounting. That is, when an environment temperature varies after the mounting, owing to differences in thermal expansion coefficients of a semiconductor and a workpiece, at a bonding portion of the semiconductor element and a solder bump, stress is generated. Making the semiconductor element thinner is intended to reduce the stress.
A mounting structure that is formed of such a thinned semiconductor element will be explained with reference to the drawings. FIG. 11A is a sectional view of an existing mounting structure and FIG. 11B is a diagram showing a deformed state of a semiconductor device in the existing mounting structure. In FIG. 11A, on base plate 10, semiconductor device 1 is mounted, and to electrode 10a formed on a top surface of base plate 10, bump 3 that is disposed on a circuit formation surface of semiconductor element 2 with solder as a formation material is bonded. Semiconductor element 2, as mentioned above, is made thinner with an intention of minimizing the stress generated at a bonding portion of the semiconductor element and the bump.
FIG. 11B shows a state where in a mounting structure in which semiconductor device 1 having such thinned semiconductor element 2 is mounted on base plate 10, thermal contraction stress is generated in reflowed base plate 10. Since semiconductor element 2 is thin and flexible, in accordance with the contraction deformation of base plate 10, semiconductor element 2 deforms accordingly. In a mounting structure where after forwarding the thinned semiconductor element 2 having a thickness of 150 μm or less is used, the deflection deformation of semiconductor element 2 shows a deflection shape (part shown with an arrow mark P1) in which semiconductor element 2 is concave between respective bumps 3; that is, as the thinning goes further, the improved traceability can be realized. It is demonstrated that thereby, a level of the stress generated at a bonding portion of semiconductor element 2 and bump 3 can be effectively reduced.
However, in a mounting structure made of thinned semiconductor element 2, disadvantages described below are confirmed empirically and according to numerical analysis. As shown in FIG. 11B, the deflection of semiconductor element 2 (shown with an arrow mark P2) rapidly increases outside bump 3 at the outermost periphery. Accordingly, in some cases, there is caused a phenomenon in that in the area outside of outermost periphery bump 3, a crack is generated on a bottom surface of semiconductor element 2, and semiconductor element 2 is broken due to the crack. That is, there is a problem in that as the semiconductor element is made thinner, while the stress generated in the solder bump is lowered, the neighborhood of the outer periphery of the semiconductor element is locally broken.